The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image areas and the water or fountain solution is preferentially retained by the nonimage areas. When a suitably prepared surface is moistened with water and an ink is then applied, the background or nonimage areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and other materials. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
A widely used type of lithographic printing plate has a light-sensitive coating applied to an aluminum base support. The coating may respond to light by having the portion that is exposed become hardened so that nonimage areas are removed in the developing process. Such a plate is referred to in the art as a negative-working printing plate. Conversely, when those portions of the coating that are exposed become soluble so that they are removed during development, the plate is referred to as a positive-working plate. In both instances, the coating remaining on the plate is ink-receptive or oleophilic and the nonimage areas or background are water-receptive or hydrophilic. The differentiation between image and nonimage areas is made in the exposure process where a film is applied to the plate with a vacuum to insure good contact. The plate is then exposed to a light source, a portion of which is composed of UV radiation. In the instance of positive-working plates, the areas on the film corresponding to the image areas are darkened, preventing light from making those plate coating areas developer soluble, while the areas on the film corresponding to the plate nonimage areas are clear, allowing them to become soluble. The solubilized plate image areas can be removed during development. The nonimage areas of a positive-working plate remain after development, are oleophilic and will accept ink while the exposed areas that have had the coating removed through the action of a developer are desensitized and are therefore hydrophilic.
Various useful printing plates that can be either negative-working or positive-working are described, for example, in GB 2,082,339 (Horsell Graphic Industries), and U.S. Pat. No. 4,927,741 (Garth et al), both of which describe imaging layers containing an o-diazoquinone and a resole resin, and optionally a novolac resin. Another plate that can be similarly used is described in U.S. Pat. No. 4,708,925 (Newman) wherein the imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. This imaging composition can also be used for the preparation of a direct laser addressable printing plate, that is imaging without the use of a photographic transparency.
Printing plates comprising imaging layers that contain novolac resins, infrared radiation absorbing compounds and other materials are described, for example, in U.S. Pat. No. 5,340,699 (Haley et al), U.S. Pat. No. 5,372,907 (Haley et al), U.S. Pat. No. 5,372,917 (Haley et al), U.S. Pat. No. 5,466,557 (Haley et al) and EP-A-0 672 954 (Eastman Kodak). Imaging with these plates includes exposure to near-infrared energy to produce acids in an imagewise fashion. These acids catalyze crosslinking of the coating in a post-exposure heating step. Precise temperature control is required in the heating step.
DE-4,426,820 (Fuji) describes a printing plate that can be imaged in the near infrared at moderate power levels with relatively simple processing requirements. This printing plate has at least two layers: an imaging layer containing an o-diazoquinone compound and an infrared radiation absorbing compound, and a protective overcoat containing a water-soluble polymer or silicone polymer. This plate is floodwise exposed with ultraviolet light to convert the o-diazoquinone to an indenecarboxylic acid, which is then imagewise decarboxylated by means of heat transferred from the infrared radiation absorbing material. Development with an alkaline solution results in removal of areas not subjected to thermal decarboxylation. The pre-imaging floodwise exposure step, however, is awkward in that it precludes the direct loading of the printing plates into plate-setters.
U.S. Pat. No. 5,705,308 (West et al.) and U.S. Pat. No. 5,705,322 (West et al.) describe infra-red radiation-sensitive, negative-working printing plates having imaging layers containing novolac resins and diazonaphthoquinones. A UV flood-exposure step is required before processing these plates. Although the UV flood exposure step has considerably more latitude than the post-exposure baking step used with the plates noted above, it would be desirable to avoid additional steps between imaging and processing.
Optical recording media having laser imageable layers are described in U.S. Pat. No. 4,966,798 (Brosius et al). Such layers contain an infrared radiation absorbing dye or pigment in a phenolic resin, and are resident on a suitable polymeric support. Recordation is carried out using a laser to bring about a surface change in the imageable layer. Printing plates are different materials and require a different imaging process.
In copending and commonly assigned U.S. Ser. No. 08/822,376 of Sheriff et al positive-working printing plates are described that can be processed directly after imaging without any intervening baking or floor exposure steps. Such plates have a very simple imaging layer consisting essentially of a novolac resin and an infrared radiation (IR) absorbing compound in specific molar ratios.
However, there is a need to increase the processing latitude of these printing plates so that the development conditions need not be so carefully controlled in order to provide desired discrimination between image and nonimage areas. Processing latitude can be increased by incorporating diazonaphthoquinones, but in order to preserve the positive-working nature of such materials, the amount of the IR absorbing compounds must be restricted below certain threshold levels. The presence of diazonapthoquinones also makes such printing plates more sensitive to room light, negating one advantage of so-called "thermal" printing plates.
Thus, there is a need for simple printing plates that can be easily imaged in the near infrared at moderate power levels and that require relatively simple processing methods. It is also desired that such printing plates have improved processing latitude without the disadvantages presented by the use of diazonaphthoquinones.